Think of them like a wedding event rental company, they are more than happy to rent you their own brand of tables, flatware, and silverware, but if you want another brand that’s fine too as long is you buy from them.
I expect they'll also try to port Citus Data functionality to the SQL Server platform.
Also PostgreSQL has improved by large margins in the last five years. The software is more akin to a pyramid rather than a skyscraper. The foundation took a long time, but now that its complete there is a strong base for growth.
It also lets teams own their DB infrastructure and play around with deployment patterns that'd make little sense on Oracle et al because of cost reasons.
It's not that Postgres is "killing" commercial databases, it's just that more and more people are, as you said, noticing that they don't need a commercial database for lots of use cases. And support and consultancy -- and even in-team skills -- for Postgres are often available.
SQL Server platform already has one of the most advanced optimizers and distributed planning with its use in Polybase, stretch tables, SQL MPP and Azure SQL DW.
Aster and Greenplum perform exceptionally for what they do. If SQL Server were better that's exactly what companies would use. Commercial engines are definitely more advanced. But some Postgres derivatives can absolutely out perform enterprise platforms in some uses-cases, and vice-versa.
However I manage roughly 2000 instances of commercial databases. I'd say maybe a 10th could not be hosted on postgres.
It gives postgres a huge disruption potential and the management, in all the big firms I know, is actively looking at it.
I'm sure PG can take on more of the standard RDBMS workloads today but I don't think that's really making a big dent on SQL Server as the bulk of their revenue comes from the serious enterprise scenarios.
I'd say postgres can take 90% of the revenues. I've worked in 3 of the top 10 european banks. Most of the SQL Server instances do not even need partitioning, for example. Let alone always on, hekaton and so forth.
People mostly buy peace of mind. Until they are charged millions and start questioning the stupid expensive bills saying "do we need that ?"
Really I have all the metrics to back this up: CPU usage/Availability requirements/data size... It is literally my job to collect those.
Also, proper window support, great postgis, great json support, open source ecosystem support are not simple and huge cost savers.
If you mean the licensing and support from commercial distributions and vendors then, as you already recognize, these decisions will fall to which vendor they trust more. That will usually end up being Microsoft.
Enterprises are more than just banks, and vendor relationships matter. They're not fungible and are rarely based on price.
15 years ago Windows Server was far more advanced than any Linux distribution. What does the server OS market look like today?
As I said, "If you can use Postgres then you didn't need a commercial system in the first place."
Microsoft doesn't regard PostgreSQL as a direct threat to SQL Server and it's happy to make money where it can so long as it doesn't perceive something to be a mortal threat.
Azure is happy to take your money to run SQL Server or Postgres, just like how Amazon has been running Aurora side-by-side with Oracle and SQL Server for years now.
Azure Database for PostgreSQL competes against RDS and to a lesser extent, Google's Cloud SQL for PostgreSQL.
They'll still sell MS SQL Server too. But sometimes PostgreSQL is a better fit for your stack, or your preference, and they want your money for them to provide that too.